ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-189-2017A high-altitude balloon experiment to probe stratospheric electric
fields from low latitudesGurubaranSubramaniangurubara@iigs.iigm.res.inShanmugamManuhttps://orcid.org/0000-0003-3307-456XJawaharKaliappanEmperumalKaliappanMahavarkarPrasannaBuduruSuneel KumarIndian Institute of Geomagnetism, Navi Mumbai 410 218, IndiaTata Institute of Fundamental Research, Mumbai 400 005, IndiaEquatorial Geophysical Research Laboratory, Indian Institute of
Geomagnetism, Tirunelveli 627 011, IndiaNational Balloon Facility, Tata Institute of Fundamental Research,
Hyderabad 500 062, IndiaSubramanian Gurubaran (gurubara@iigs.iigm.res.in)3February201735218920131July201611January201712January2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/189/2017/angeo-35-189-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/189/2017/angeo-35-189-2017.pdf
The Earth's electrical environment hosts a giant electrical circuit, often
referred to as the global electric circuit (GEC), linking the various sources
of electrical generators located in the lower atmosphere, the ionosphere and
the magnetosphere. The middle atmosphere (stratosphere and mesosphere) has
been traditionally believed to be passively transmitting electric fields
generated elsewhere. Some observations have reported anomalously large electric
fields at these altitudes, and the scientific community has had to revisit the
earlier hypothesis time and again. At stratospheric altitudes and especially
at low latitudes, horizontal electric fields are believed to be of
ionospheric origin. Though measurements of these fields from a balloon
platform are challenging because of their small magnitudes (around a few
mV m-1), a suitably
designed long-duration balloon experiment capable of detecting such small
fields can provide useful information on the time evolution of ionospheric
electric fields, which is otherwise possible only using radar or satellite
in situ measurements. We present herein details of one such experiment, BEENS
(Balloon Experiment on the Electrodynamics of Near Space), carried out from a
low-latitude site in India. The instrument package for this experiment is
comprised of four deployable booms for measurements of horizontal electric
fields and one inclined boom for vertical electric field measurements, all
equipped with conducting spheres at the tip. The experiment was conducted
from Hyderabad (17.5∘ N, 78.6∘ E) during the post-midnight
hours on 14 December 2013. In spite of a few shortcomings we report herein, a
noticeable feature of the observations has been the detection of horizontal
electric fields of ∼ 5 mV m-1 at the stratospheric altitudes of
∼ 35 km.
Ionosphere (electric fields and currents)Introduction
The various sources of the electrical generators located in the lower atmosphere, the ionosphere and the magnetosphere and the currents flowing through them as well as between them and Earth constitute a giant electrical circuit often referred to as the global electric circuit (GEC; see Rycroft et al.,
2012, for recent advances on this topic). The classical picture of the GEC
incorporates the global thunderstorm activity as the only source, with the
currents flowing from the thunderclouds charging the ionosphere to a few
100 kV with respect to the Earth and the discharging currents leaking
through the atmosphere over rest of the region. It was recognized some time
ago that there are at least two other important sources that contribute to
the global electric fields, namely the solar wind–magnetosphere dynamo that
generates potential drop across the northern and southern polar ionospheric
caps in the range of 30–150 kV and the ionospheric dynamo originating in
the 100–150 km altitude range due to tidal forcing that produces potential
differences of 5–10 kV (Hays and Roble, 1979a, b).
In the past, the middle atmosphere was often considered to be
electrodynamically passive, so that electric fields observed at stratospheric
and mesospheric altitudes could be interpreted as those penetrating from
above or from below. Solar-wind-induced magnetospheric convection, for
example, produces large-scale fields of several tens of mV m-1 in the
polar cap and in the auroral ionosphere. These horizontal fields were indeed
observed at balloon altitudes with little attenuation (see Mozer and Serlin,
1969, for example) in accordance with the results from a theoretical analysis
(Park, 1976). In addition, large-scale ionospheric dynamo fields of a few
mV m-1 are expected to penetrate to stratospheric altitudes with little
attenuation (Volland, 1972). It is to be noted that the global thunderstorm
activity driving fair-weather electric fields is observed as vertical fields,
whereas the upper atmospheric sources induce horizontal fields at balloon
altitudes. In the case of the former, the Earth and the ionosphere act as
equipotential surfaces, and the sources drive electric fields, which point
essentially downward in the fair-weather regions, i.e., far away from
thunderstorm centers. In the case of the latter, the two upper atmospheric
generators, namely the solar wind–magnetosphere–ionosphere dynamo and the
ionospheric neutral wind dynamo, produce charge separation, resulting in
large-scale electric fields, which are primarily in the horizontal direction.
These horizontal fields map downward, as they are, to mesospheric and
stratospheric altitudes.
Among the recent additions to our knowledge of the above processes, one
spectacular phenomenon that deserves mention is the high-altitude discharge
and the corresponding luminous events occurring above thunderstorms (Franz et
al., 1990; Sentman et al., 1995; Rycroft and Odzimek, 2010, to state a few).
Holzworth (1989) discovered the existence of large horizontal electric fields
at stratospheric altitudes (∼ 27 km) from long-duration flights
involving superpressure balloons. Until then, it was believed that the middle
atmosphere essentially behaved as a passive element transmitting electric
fields of lower and upper atmospheric sources. A later report by Hu and
Holzworth (1997) confirmed the presence of these large electric fields over a
wider latitude region of 30–75∘ in the Southern Hemisphere. It was
hypothesized then that atmospheric motions driven by an inertia gravity wave
originating in the lower atmosphere could effectively induce horizontal
electric fields in the range 10–20 mV m-1 during the night and
100–150 mV m-1 during the day, with the electric field vector undergoing
a rotation at a rate equal to the period of the local inertia gravity wave.
For inertia gravity waves with horizontal wavelengths of ∼ 500 km, it
follows that a corresponding inertial cell of size 500 km is capable of
generating a 100 mV m-1 electric field, which would lead to a total
potential drop of ∼ 50 kV across the cell that would be comparable to
the potential drop across the entire magnetosphere (Holzworth, 1995).
The first experiment of the Balloon Experiment on the Electrodynamics of Near
Space (BEENS) carried out from the National Balloon Facility (NBF), Tata
Institute of Fundamental Research (TIFR), Hyderabad (17.5∘ N,
78.6∘ E), India, was of exploratory type with the primary objective
of capturing electric field signals at stratospheric altitudes from low
geomagnetic latitudes (∼ 8–9∘). It is generally believed that
fields other than those driven by thunderstorm sources in the lower
atmosphere are weak (1–3 mV m-1) and are of ionospheric origin at
such low geomagnetic latitudes. Advances in femtoampere bias current
field-effect transistor amplifier technology and some simple circuit ideas that emerged
to address the problem of minimizing the leakage currents had motivated us to
carry out measurements of such weak electric fields. The experiment was
conducted during the early hours of 14 December 2013, using a
110 000 m3 balloon that reached a float altitude of 35 km.
In Sect. 2, a brief description of the BEENS experiment and the
instrumentation involved is presented. Sketches showing the gondola, as well
as the central body, and the alignment of probes as per their configuration
chosen for this experiment are provided. Payload electronics are discussed in
this section with the help of a schematic showing the major circuit elements
and the signal flow. As shown in this section, a thermal infrared image
available from the EUMETCast data archival center revealed fair-weather
conditions across the Indian Peninsula at the time of launch. The first
results from this experiment are presented in Sect. 3. Reliable measurements
of vertical electric field were not possible due to the five-probe
configuration and the kind of deployable boom incorporated for the vertical
probe, thereby leaving us with a meaningful interpretation to be made with
the measurements from the horizontal probes alone. During the flight, we also
encountered change in the DC offset in the electric field measurements and
enhancement in signal amplitudes possibly caused by the DC torque motor used
for spinning the payload. These aspects are discussed in this section.
Finally, in Sect. 4 we present a discussion on the possible sources of the
observed electric fields.
Description of the BEENS experiment
Employing the double-probe technique, which is well discussed in the
literature (e.g., Holzworth and Bering III, 1998, and references therein),
the BEENS payload mounted on a gondola comprised two orthogonal pairs of
spherical probes, namely P1–P2 (referred to as x probes) and
P3–P4 (referred to as y probes), with the line joining the
probes P1 and P2 defining the payload x axis and the line joining
the probes P3 and P4 defining the payload y axis (see Fig. 1a for
the geometry of the probe alignment). The reference magnetometer was mounted
on the top deck of the central body in such a manner that its two horizontal
sensors were aligned along the two horizontal payload axes defined above. The
probes deployed in the horizontal plane were configured to yield measures of
the north–south and east–west components of the horizontal electric field.
The fifth probe (P5) was mounted at the end of another boom that was
deployed at an angle of 45∘ to the horizontal plane. In this manner,
this probe, along with one of the probes deployed in the payload x axis,
namely P1, was configured to yield a measure of the vertical electric
field (Ez) (the pair P5–P1 will hereafter be referred
to as z probes).
(a) A sketch of gondola (central body or the payload) and
the booms shown in deployed condition (not to scale). The directions of the
three axes referenced with respect to the probe configuration are indicated.
The curved line with an arrow at its tip indicates the direction of the
rotation of gondola. The dimensions of the gondola are given in (b).
Details regarding the individual lengths of the booms are provided in the
text. (b) A drawing of the gondola showing the locations of various
subsystems (courtesy of the National Balloon Facility, TIFR, Hyderabad).
All five probes were identical 20 cm diameter aluminum spheres coated with
Aquadag, a conducting carbon paint used to minimize the work function
differences between them. Each of the four booms to where the horizontal x
and y probes were mounted was 3 m long, with its outermost 1 m made
of insulating fiberglass rods and was kept in stowed condition prior to the
launch. With the gondola dimensions of about 1 m each on all sides (see
Fig. 1b for a drawing of the gondola showing the locations of the various
subsystems), the probe-to-probe baselines achieved for the x and y probes
were 6.92 and 7.02 m, respectively. The probe-to-probe baseline achieved in
the vertical direction was 2.82 m.
A DC torque motor, referred to hereafter as orientor, enabled the rotation
(clockwise) of the gondola about the vertical axis for meaningful horizontal
electric field measurements. A universal coupler immediately following the
orientor provided additional stability for the platform by ensuring that the
axis of rotation did not get tilted as the balloon maneuvred through and
around the ambient medium during its flight. Ground tests prior to launch
yielded a rotation rate in the range of 1–2 rpm. The orientor did not have
a control arrangement for this experiment, and so it was anticipated that the
rotation rate would increase as the balloon ascended to higher altitudes (due
to lower friction with the ambient gas higher up), which indeed was the case.
Figure 2 depicts a photograph of the payload undergoing rigorous field tests
with the booms having been kept in the deployed condition.
A photograph showing the payload undergoing field tests. The payload
rotation was carried out for several hours and the stability of the platform
was ensured during these tests.
A
block diagram of the experimental setup of the BEENS payload.
The three-axis fluxgate magnetometer (FGM) serving as a reference magnetometer or
magnetic compass for the electric field measurements as mentioned earlier and
a two-axis search coil magnetometer (SCM) for monitoring fluctuating magnetic
fields in the ELF–VLF frequency range completed the instrument package. The
search coil sensors were mounted at the end of a rigid 1.5 m long boom. The
four sides (of ∼ 1 m2 area) of the gondola were mostly covered by
aluminum panels with Teflon spacers, which electrically isolated the panels
from the gondola. These aluminum panels were also coated with Aquadag. With
their large conductive area (∼ 3.6 m2), the panels provided a
common ground. Considering that the covered area was 2 orders of magnitude greater than
the probe surface area (∼ 0.0314 m2), it was felt that this would
be sufficient enough for a reliable measurement of electric field.
Figure 3 depicts a block diagram of the experimental setup of the BEENS
payload. A specially designed three-channel differential electrometer with
ultra-high-input impedance permitting ultra-low-input bias current
(∼ 3 fA at 25 ∘C) was used to sense the difference voltages
between the probe pairs. Shielded cables brought the high-impedance input
signal directly to the inputs pins of an INA116 instrumentation amplifier
operated in a differential mode. Guard voltages were inherently generated
within the instrumentation amplifier and were available for driving the outer
conductors of the shielded cables to the floating potential of the
corresponding probe. This arrangement combined with the way the wires were
soldered directly in air on to the input pins of INA116 amplifier enabled
considerable reduction of any stray parasitic capacitance along the signal
path. Further, with a single external resistor, a corresponding gain setting
could be achieved with this instrumentation amplifier. Any circuit
complications were greatly minimized with the optimum use of a single
instrumentation amplifier for each of the electric field channels, which
would have the advantage of carrying out all three functions mentioned above,
i.e., differencing, generating the guard voltage and providing a gain setting
(see Jawahar et al., 2016, for further details of the electrometer circuit).
One of the design goals of the above circuit was to capture high-frequency
(a few Hz to few tens of kHz in the ELF and lower VLF range) signals, in
addition to the slowly varying DC electric field. To meet this requirement,
provision was made for a second-order low-pass filter with cutoff frequency
that could be chosen to be 1 or 32 kHz depending on the frequency range of
interest. Further, the single-board version of an embedded control and
acquisition unit (referred to as a single-board reconfigurable input/output
(SBRIO) device) of National Instruments was included as part of the
instrument package to record high-frequency data (refer to Fig. 3). The SBRIO
could monitor and record high frequency electric field signals with its
sampling rate of 200 kS s-1 and onboard storage option. The
electrometer output was simultaneously routed to the 12 bit ADC input of an
onboard telemetry encoder. In this work, we examine only the telemetered
double-probe data collected at slow sampling rates, which would essentially
represent the DC electric field at the probing balloon altitudes.
Within the flight encoder, the output from the three electric field and the
three magnetic field channels was converted to a single serial digital data
stream for transmission to the ground station. With a differential input in
the range of +4.7 V, the 12 bit ADC resolution would enable the
measurement system to record a change of ∼ 2.3 mV minimum (equivalent
to a horizontal electric field of ∼ 0.3 mV m-1). The raw data
were sampled at ∼ 2.25 ms time interval. The encoder was programmed to
transmit 32-channel serial data, which, therefore, resulted in every channel
being sampled at a ∼ 72 ms time interval (or 13.8 Hz sampling
frequency). As mentioned earlier, the rotation of the payload would induce a
sinusoidal signal of the electric field at a period equal to the payload
rotation period. On this count, the sampling frequency of 13.8 Hz for the
electric field channels noted above would be sufficient given that the
smallest gondola rotation rate was 0.16 Hz (corresponding to a period of
6 s), as we will see later.
Ground verification of the payload electronics was carried out on the ground in
advance, wherein each pair of probes was tested by applying DC potentials
(one of the probes was given positive voltage and the other grounded) and the
output monitored on the corresponding telemetry channel. The results from one
such test performed for the x and y probes just prior to launch are
presented in Fig. 4. The linear response of the electrometer output for
various probe voltages is clearly evident in Fig. 4, indicating the
anticipated performance of the electric field channels prior to launch. We
need to mention here that, apart from ground tests and verification, there was
no provision made in the present experiment for onboard calibration of the
probe–electrometer system.
Performance results for the probe–electrometer system obtained just
prior to launch. Known voltages are applied to the x and y probes
separately in isolation and the electrometer outputs monitored on the
respective telemetry channels. The equation for the linear regression line
and the correlation coefficient (R) are mentioned separately in each plot.
A load line of about 75 m was used to connect the balloon with the payload.
The main packages along the load line were a cut-off device situated just
below the balloon, a parachute, a GPS radiosonde and a radar reflector. The
support packages for the BEENS experiment were the telemetry unit, a
telecommand unit, a ballast container, a battery pack and other housekeeping
devices. The launch was cleared after critical assessments of the functioning
of all subsystems, including the payload, were carried out through continuous
long-duration ground tests.
It was expected that the effects of any electrical noise created by the
balloon movement through the ambient atmosphere would be overcome by the
long load line and the electrical isolation of the probes. Further, any
balloon-induced noise would appear as common-mode noise, which would be
eliminated because of the differential measurements.
Several flights have been conducted in the past from the same location
during the Indian Middle Atmosphere Program in the 1980s (Gupta and Narayan,
1987; Garg et al., 1989; Sampath et al., 1989; Udare et al., 1991). Most of
those flights were aimed at making measurements of polar conductivities, ion
densities and mobilities, though a few of them yielded vertical electric
field profiles during the balloon ascents.
The balloon launch with the BEENS payload took place at 00:50 Indian Standard
Time (IST), which is 5.5 h ahead of Greenwich Mean Time, on
14 December 2013, after being postponed twice on two subsequent days due to
unfavorable surface wind conditions. Immediately after the release of the
balloon, a boom deployment command was given and all five booms were
successfully deployed one after another. Another command given 16 min after
the launch activated the orientor and the payload started rotating as
evidenced in the reference magnetometer data telemetered to the ground. In
Fig. 5, we present a snapshot of the payload during a late stage (early
morning hours) of the flight with booms seen in the deployed state.
A snapshot of the payload viewed from the load line hanging from the
balloon.
The orography, specific to the track along which the balloon drifted, is
contributed primarily by the Deccan Plateau, a region elevated in the range
of 300–500 m above mean sea level. There are no mountain ranges with
altitudes greater than 1000 m in the vicinity of at least 300 km from the
balloon track on all sides. The EUMETCast data archival center provided
access to the image archives from the EUMETSAT fleet of satellites. A thermal
infrared (10.2–12.5 µm) image of the Indian subcontinent obtained
from one such archive is depicted in Fig. 6. This image, which was captured
at 02:30 IST on 14 December 2013, does not reveal any high-altitude
convective cloud system in the vicinity of the balloon path (shown as a thick
red curve).
A thermal infrared image (10.5–12.5 mm) obtained from the
EUMETSAT's fleet of satellites at 02:30 IST on 14 December 2013. The balloon
track is indicated in red.
When the balloon reached the float altitude of ∼ 35 km, the
stratospheric winds in that region were blowing with speeds of
∼ 20 m s-1 towards west. With the safe flight corridor for the
recovery of the payload being about 400 km on the west, the flight had to be
terminated at around 08:00 IST after achieving a float duration of slightly
more than 4 h. Figure 7 depicts the GPS tracked balloon altitude (bottom
panel) and the wind speed and wind direction measured by the GPS radiosonde
(top panel). As noticed herein, the wind direction remained at
∼ 90∘ (which is easterly) between 03:30 and 08:00 IST, when the
balloon was at the float altitude. The westward drift of the balloon
continuously over time (shown as red curve in Fig. 6) was essentially due to
the easterly winds prevailing during this period at high altitudes.
As mentioned earlier, there was a provision kept to change the gain of the
electrometer for any of the three channels by the choice of a suitable
resistor through a telecommand. As soon as the balloon approached the float
altitude, a command was given from the ground for the x and
y electrometer voltage gain to increase by 10 times. The change was
immediately noticed in the telemetry channels. Until then, little variation in
the signal level was seen on the computer monitor in real time. Immediately
after the implementation of a 10-fold gain, the two probe outputs started
displaying variations that appeared to be in tune with the rotation of the
payload. Further, as the balloon approached its float altitude, clear
sinusoid-like variations could be seen on the real-time display unit,
indicating the presence of an ambient stratospheric electric field.
Deduction of the horizontal electric field at balloon altitudes first
involves careful examination of raw data for every payload rotation. The
magnetometer sensors were configured in such a manner that the x sensor
output led the y sensor output when the payload rotated in a clockwise
direction. It should be noted here that the labels x and y were nothing to
do with the direction (north–south/east–west) and were arbitrarily assigned
to the pairs of probes for the given geometry. Figure 8 depicts a 40 s
sample of the magnetic data (top panel) revealing that the x channel led
the y channel by 90∘ (one-fourth of the payload rotation period). In
terms of magnetic field units, the output range of 1.148–3.998 V in Fig. 8
(top panel) was referenced to 2.5 V and hence would correspond to a range of
-1.352 to 1.498 V. As the sensitivity of the magnetometers was
36.5 µV nT-1, a full range of 2.85 V peak to peak
corresponds to a magnetic field in the neighborhood of 78 082 nT
peak to peak or an amplitude equivalent to 39 041 nT, which is roughly the
value of the Earth's magnetic field at this location.
The location of the BEENS payload, when plotted altitude versus
time, during the experiment (bottom panel) and the corresponding ambient wind
speed and wind direction recorded then (top panel).
The raw magnetic field data, Bx and By from the
x and y sensors, respectively (top panel), and the corresponding electric
field channels.
With regard to the electric field measurement, as mentioned in Sect. 2, the
y probes were aligned parallel to the y sensor of the magnetometer,
whereas the x probes were aligned anti-parallel to the x magnetometer
sensor. Due to this arrangement, the measurement of the x probes would have
to be shifted in phase by 180∘. In any case, the payload rotation
ensured that the ambient electric field appeared as a sinusoidal signal as
can be seen in the telemetry data for the electric field probes shown in
Fig. 8 (bottom panel). The continuous vertical line indicates the near
alignment of the y channels of electric and magnetic field probes, whereas
the dashed line reveals the near 180∘ phase difference between the
respective x channels mentioned above. For the data from the electric field
channels plotted in Fig. 8, we had used 25-point running means of the raw
telemetry outputs. Further description of the data will be presented in the
next section.
We use the traditional harmonic analysis and the local version of the
harmonic analysis, namely the complex demodulation technique, for the
estimation of horizontal electric field from the BEENS telemetry data. For
every payload rotation, the time period is determined from the magnetometer
data and the harmonic analysis is applied to the electric field data to
estimate the amplitude and phase of the horizontal electric field in a least-squares sense. The phase, computed separately for the x and y probe
outputs, would represent the direction of the electric field vector. The
magnetometer x channel would serve as a reference for the estimation of phase
from x probe outputs, whereas the magnetometer y channel would serve as a
reference for the phases estimated from y probe outputs.
The complex demodulation technique is more flexible when it comes to data for
which the payload rotation rate or the amplitude or the phase varied and when
the data cannot be expressed simply in terms of cosine functions. The complex
demodulation has been recognized to be the local version of the harmonic
analysis (Bloomfield, 2000). The analysis adopted for the present analysis
invokes demodulation at a chosen center frequency followed by low-pass
filtering. The method is well discussed in Bloomfield (2000) and will not be
repeated here.
Results
In this section we examine the behavior of the three components of the
stratospheric electric fields. As mentioned earlier, the spinning of the
payload about its vertical axis enabled useful determination of the
horizontal components of the electric field, as any differences in contact
potentials between the probes and DC offsets, if any, would appear as
quasi-DC shifts in the probe outputs. The local horizontal electric field
vector would then appear as a quasi-sinusoidal variation in each of the two
orthogonal probe outputs with a period exactly equal to the period of
rotation of the gondola about its axis.
The temporal variation in the two orthogonal components (recorded as
sinusoids) of the ambient magnetic field recovered from the reference
magnetometer data had a sinusoidal nature, and this was used to compute the
payload rotation rate. As can be noted in Fig. 9, the payload rotation
stabilized only after the balloon attained the float altitude of
∼ 35 km at ∼ 03:30 IST. The period of rotation reached the
lowest value of ∼ 6 s (corresponding to ∼ 10 rpm), increasing to around ∼ 60 s at 02:00 IST. Unfortunately, the orientor responsible
for the payload rotation malfunctioned at 05:30 IST, following which the
rotation was disabled. For these reasons, we considered only about 90 min of
data, i.e., between 04:00 and 05:30 IST, for estimating the horizontal
component of the electric field.
The payload rotation rate (squared symbols) computed using the
reference magnetometer data and plotted along with the balloon altitude
(continuous curve).
Before we present our results, a few points regarding the impact of the
five-probe configuration and the inclined boom on the veracity of the
vertical electric field measurements will be in order. The boom that was used
to deploy P5 (refer to Fig. 1a) did not have an insulating component at
its outer end like the ones used for the other four probes, though the entire
boom was electrically isolated from the gondola and the ground panels with
Teflon spacers. The potential differences measured in the vertical direction
were later found to be greatly influenced by this arrangement. Figure 10
depicts the z probe difference output for times when the balloon ascended.
We should note here that the z pair of probes could not be calibrated on
ground with known potentials as was done for the horizontal pair of probes
due to the large ambient atmospheric vertical electric field present. At this
stage, we do not intend to take the discussion about the vertical electric
field measurements any further for the simple reason that they are not
reliable.
Difference output from the z probes (P5–P1 pair).
Another issue pertaining to the measurements was to do with the requirement
of the probe geometry, wherein one of the x probes was to be used for the
vertical electric field measurement. In fact, out of the two x probes
(P1 and P2), probe P1 was reckoned to measure a -Ve
potential for the P5–P1 pair and therefore connected to the
negative input of the z instrumentation amplifier and was reckoned to
measure a +Ve potential for the P1–P2 pair and connected to the
positive input of the x instrumentation amplifier. As can be appreciated,
there was an interconnection that was to be realized between the inputs of
x and z channels. With this arrangement, any gondola noise present in the
z channel was likely to couple to the x channel. In Fig. 11, we show the
nature and extent of coupling of these two channels for a data set spanning
2 min between 03:46 and 03:48 IST. No discernible electric field signature
varying at the gondola rotation rate can be inferred from the x channel at
these times. Rather, the x channel is seen to track the z channel output.
The inadvertent manner in which the electrical interconnections were realized
for this five-probe configuration resulted in the cross-coupling between the x
and z channels, implying that the derived amplitudes of the ambient electric
field would be underestimated to a large extent. However, about 30 min
later, when the balloon reached a stable float altitude (refer to Fig. 9), the
x channel showed sinusoidal-like variations but with reduced amplitudes for
reasons not known yet. In contrast, the y probes were isolated electrically
and distanced well away from the booms used to deploy other probes (refer to
Figs. 1a and 5). Because one pair of probes is sufficient for estimating
horizontal electric fields on a rotating platform, we consider only the
y channel telemetry output for that purpose in the rest of the paper.
Before we examine the data sets, we caution the readers about a possible
electrical/electromagnetic noise caused by the DC torque motor and the power
supply driving the motor, which could have impacted the measurements. At
03:48 IST, the power supply to the orientor was switched off by a ground
command (at 03:48 IST) for a brief interval of ∼ 3 min to assess the
performance of the electric field channels then. The magnetometer output
revealed that the payload continued to spin at the same rate due to its
inertia, though the supply to the motor was switched off. During this time
interval, the electric field channels registered a considerable change in the
DC offset and we noticed nearly a 2-fold decrease in the signal amplitude in
the y channel.
The raw outputs from the x, y and z probe
pairs (b–d) along with the magnetometer x channel
output (a).
In Fig. 12, we show the y channel output, wherein we depict the 25-point
running means plotted in electric field units of mV m-1, along with the
demodulated signal representing the electric field. The electric field values
were obtained after dividing the difference output voltage by the physical
distance between the corresponding probes. It can be readily seen that the DC
offset of this channel decreased from ∼ 45 to ∼ 5 mV m-1.
Further, the amplitude of the electric field signal was reduced from
∼ 8 to ∼ 5 mV m-1 during this period, as can be seen in the
bottom panel of Fig. 12. As soon as the power supply to the DC torque motor
was switched on, both DC offset and the signal amplitude reached their
earlier values (not shown here). We do not know yet how the motor or its
power supply could have caused the subtle variations noticed in the data. On
the other hand, it makes sense to treat the difference output at times when
the motor was switched off as free from the motor-induced noise and use them
to scale down the electric field values just outside this time interval.
Further, the scaling factor of ∼ 2.4 so obtained was used for the
remaining data set for the entire 90 min duration. In this process it is
assumed that the phases did not vary, though the amplitudes were scaled down.
Sample electric field data at the float altitude of ∼ 35 km for three
different 2 min time segments, namely 03:37–03:39, 04:37–04:39 and
05:25–05:27 IST, are depicted in Fig. 13. The data points have been
smoothed with 25-point running means that correspond to windows of 1.8 s
time duration. The electric field modulated by the payload rotation can be
clearly seen in these data sets. Before proceeding to estimate the electric
fields, the raw data for the above duration were filtered with a pass band in
the range of 5.6–7.3 s, corresponding to the range over which the payload
rotation period varied. With this process, the DC offsets and any of their
slowly varying components were removed in advance.
The Ey channel output (top panel) (converted to electric
field units) for a brief interval of time when the DC torque motor was
switched off. In the bottom panel, the demodulated signal is shown. The
dashed vertical line represents the time (03:48 IST) when the DC torque
motor was turned off. For further details, see the text.
We now elaborate the procedure adopted for meaningful interpretation of the
amplitude and phase of the electric field vector estimated for every payload
rotation or cycle. The first step involved the use of the magnetometer
y channel data to compute the period of the payload rotation for a
particular instant. The same data set also served to denote the reference
time for the electric field channels for the corresponding cycle. In specific
terms, the positive peak of the magnetometer y channel data would define
the beginning of the time or “time zero” or, equivalently, the position of
the gondola with its angular measure equal to 0∘ for that particular
cycle and the subsequent times for the respective cycle would be reckoned
with reference to that instant. The following magnetometer y channel peak
would define the “time zero” for the next cycle and so on. The electric
field data for every cycle, with time so defined, were separately analyzed
using the harmonic analysis, and the amplitudes and phases were estimated in
the least-squares sense. As per the convention adopted, the phase of the
electric field vector would refer to the time when the electric field reaches
its maximum value within a particular cycle. While depicting the results, we
convert the time to angular measure (clockwise from geomagnetic north),
reckoning one payload cycle to cover 360∘. With geomagnetic north as
a
reference, the phase of the electric field vector thus defines the position
of the electric field vector in space. We repeat the above procedure
separately for the x probe outputs but with magnetometer x channel
serving as a reference.
Sample electric field data at the float altitude of 35 km for the
three different 2 min time segments. The DC offsets were removed in this
plot.
In Fig. 14, the amplitude (panel a) and phases (panel b) of the electric
field estimated from the band-pass-filtered data for the y probe outputs
over the selected time interval are depicted. As mentioned earlier, this
90 min duration was chosen considering the fact that the payload rotation
during this period was stable and the rotation rate was nearly constant.
There were ∼ 600 payload rotations during this period, with each
rotation leading to a measurement of electric field vector in the horizontal
plane of the payload. The amplitude of the electric field estimated from the
y probes was in the range of 4–6 mV m-1 during the period
considered for analysis.
Though the amplitudes derived from the x probe outputs (not shown here)
were underestimated due to the distortion induced by the z probe, the
phases for the x channel were stable over the last 1 h observation
duration, as can be seen in the bottom panel of Fig. 14. It may be noted
that both x and y channel phases were referenced with respect to
geomagnetic north and therefore indicate the direction that the electric
field vector would point at any instant of time. The observation that both
x and y probes measured nearly the same phase confirms that the x and
y probes were indeed sampling the same ambient electric field.
An interesting inference to be made from the bottom panel depicted in Fig. 14
is that the electric field was predominantly in the zonal direction and
directed towards west before 04:30 IST, when their “phases” were around
270∘. The electric field turned clockwise later between 04:30 and
05:00 IST, and was directed northwestward with their “phases” showing
values around 330∘. When the x probe outputs were more stable
during the last 1 h of useful electric field observation, one can notice
similar behavior, and it was therefore consistent with the direction of the
electric field inferred from the y probe output.
Finally, we show the geomagnetic north–south (ENS) and
geomagnetic east–west (EEW) components of the electric field
vector computed from the y probe outputs for the 90 min duration in
Fig. 15. These were the 5 min averages of the estimated electric field
components with the error bars representing their standard deviation within
the respective 5 min block. The electric field was largely westward until
04:30 IST but later changed direction to northwestward. In Fig. 15 we notice
∼ 5 mV m-1 northward field and somewhat weaker
∼ 3 mV m-1 eastward field, with the standard deviations in the
range of 0.5–1.0 mV m-1.
The magnitude and direction (a, b, respectively) of
horizontal electric field at stratospheric altitudes derived from y probes
obtained from every payload spin duration between 04:12 and 05:30 IST. In
the bottom panel, the phases derived from x probes are also shown for
comparison. Data from the x probes prior to 04:30 IST were noisy and phase
estimates for those times are not shown.
Discussion
As mentioned earlier, the intended goal of the first BEENS experiment was to
demonstrate that weak horizontal electric fields at stratospheric altitudes
could be captured. A unique configuration of the geomagnetic field at the
Equator permits a variety of electrodynamical phenomena to occur in the E and
F regions of the ionosphere. The equatorial ionosphere is characterized by
major features like the plasma fountain responsible for the large-scale
redistribution of the ionization during daytime, generation of a spectrum of
ionospheric irregularities in the dusk sector and the development of intense
current system during daytime called the equatorial electrojet (Kelley, 2009, and
references therein), all of which are of significant interest to the
scientific community and whose understanding contributes to the subject
matter of space weather. As electric fields are the major drivers for these
phenomena, the importance of monitoring them cannot be overemphasized.
The payload for the BEENS experiment was designed with the presumption that
electric fields at low latitudes were expected to be small (around a few
mV m-1). The design envisaged the use of 3 m long booms for the
horizontal probes with a probe-to-probe separation of ∼ 7 m. For the
outer 1 m portion of each of the horizontal booms, insulating fiberglass
rods were to be used. However, for the inclined boom used to support the
probe intended for vertical electric field measurements, no such insulating
arm was proposed. Along with the probe configuration that permitted use of
five probes instead of six, implementation of this design had placed
constraints on the usefulness of the electric field estimates obtained from
x and z probes. There were issues with the motor used for spinning the
payload that restricted useful electric field measurements to less than 2 h
out of a possible 4.5 h at the float altitude. Also, when the power supply
to the motor was cut off for a brief interval, we detected a perceptible
shift in the DC offset and a reduction in the amplitude of the difference
output. The motor, when powered, thus possibly induced an additional response
in each of the electrometer channels. The authors intend to address these
deficiencies and shortcomings in the next flight. Further, any noise
environment around the motor and its power supply will have to be examined
carefully before launch, and remedial measures are to be taken in order to
minimize any electromagnetic noise emanating from them.
The 5 min averages of electric field components derived from the
horizontal probe outputs, the northward component or ENS
plotted in the top panel and the eastward component or EEW
plotted in the bottom panel. The error bars represent the standard deviation
for the electric field estimates made for the individual 5 min blocks. It
should be noted that the east–west and north–south directions are referenced
in the geomagnetic frame.
We now turn our attention towards the electric field measurements reported in
this work. The modulation of the y probe difference output by the payload
rotation is quite clear during the 90 min duration considered for analysis
when the rotation rate was uniform (see Figs. 9 and 13). We believe that
unlike x probes the electric field estimates obtained from the y probes
were not influenced by the deployable boom configuration chosen for the
z probes. It may be noted that the latter might contribute to a distortion
of the field around the x probe (P1) underneath the z probe
(P5) but not around either of the y probes (P3 and P4) (see
Fig. 1a for the probe geometry). Probes P3 and P4 were well away
and were electrically isolated from rest of the probes and their supporting
structures.
During the flight, the power to the DC torque motor was cut off for a brief
time interval (∼ 3 min) as an exercise to see whether the gondola
would slow down in its spinning motion and whether there would be an impact
on the electric field measurements. During the post-launch analysis, it was
noticed that the difference output in all three channels did change. However,
the electric field signal recorded in the x and y channels retained its
character except for a reduction in the amplitude. We used the electric field
values estimated for times when the power supply was turned off (the payload
continued to spin, though) in order to scale the electric field during rest of the
times. The recalculated electric field was in the range 4–6 mV m-1. A
noticeable feature of the observations was the similar behavior of the
phases estimated from the x and y probe outputs (refer to Fig. 14), which gave us confidence in the measurements reported herein. Reaffirming that
electric field measurements of ∼ 5 mV m-1 reported in this work
are real, we now proceed to examine the source of the electric field measured
by the pair of y probes.
Observing horizontal fields of ionospheric wind dynamo origin from a balloon
platform at low to midlatitudes has always been a challenge, because they
are smaller (around a few mV m-1) and are difficult to measure.
Gonzalez et al. (1982) reported large horizontal electric fields (up to
10 mV m-1) in the Brazilian Magnetic Anomaly region. To explain this
feature, they invoked the idea that energetic particle precipitation,
associated with enhanced magnetic activity, induced irregularities in middle-atmospheric conductivity that in turn produced distortion in the fair-weather
vertical electric field and a corresponding change in the horizontal electric
field. Orographic features have been known to produce similar irregularities
in the conductivity profile that are capable of explaining large horizontal
fields observed on some occasions (Ogawa et al., 1975, for example). Such
large induced fields would clearly overshadow the weak ionospheric fields
expected at balloon altitudes.
As noted earlier, there are no marked orographic features in the vicinity of
the balloon track, and therefore we do not expect any enhanced electric field
of orographic nature to appear in the experimental data. Further, the results
presented in this work pertain to the float altitude of ∼ 35 km, well
above the altitude region over which orographic effects are expected to be
pronounced. Also, fair-weather conditions prevailed throughout the experiment
duration on the ground and just above the ground. The Kalpana satellite
imagery does not reveal any high-altitude convective cloud system over the
southern Indian peninsular region at the time of the balloon observations
reported herein, nor do the infrared images obtained from the EUMETSAT fleet
of satellites (refer to Fig. 6). No orographic or local thunderstorm activity
effects seemed to have contributed to the observed electric field signatures.
Further, there was low to moderate geomagnetic activity around these times.
The 3-hourly Kp index registered a value of 1- and the Dst index was in
the range of 20–25 nT (positive values) during the corresponding time
interval. The ground geomagnetic records from low latitudes revealed weak
pulsations (2–4 nT) in the period range of 10–20 min. However, as the
geomagnetic activity picked up several hours later, it was unlikely that the
observed electric fields were related to the geomagnetic activity.
Moving on to atmospheric sources, Holzworth (1989) first reported large
horizontal electric fields at balloon altitudes from campaigns conducted
during the austral summers of 1983 and 1984 involving long-duration
superpressure balloons. The detected fields were in the range of
10–20 mV m-1 during nighttime and 30–70 mV m-1 during daytime
at altitudes of ∼ 27 km. Holzworth (1989) referred to such fields as
HITS, implying electric fields that were horizontal, inertial, turbulent and
stratospheric. A striking feature of those fields was that the horizontal
vector rotated counter-clockwise at a period that matched the inertial period
of a class of atmospheric gravity waves present at those latitudes. The
existence of such fields was confirmed in a later work of Hu and
Holzworth (1997) that was carried out from an extended data set covering a
wide range of latitudes from the South Pole to 28∘ S.
As quiet-time ionospheric electric fields at low latitudes are expected to be
in the range of 1–3 mV m-1, we suggest atmospheric gravity waves to
be a potential source for the generation of electric fields of magnitudes in
the range of 4–6 mV m-1 reported in this work. Indeed, horizontal
winds driven by atmospheric gravity waves can transport charges, and in this
process polarization electric fields can be generated in order for the
current to vanish. Following the arguments presented in Holzworth (1989) and
D'Angelo (1990), the scenario can be elaborated as follows.
Because the vertical electric field, which is part of the GEC, has a gradient
with altitude, there arises a net charge density. Use of Gauss's law and
taking into account the exponential decrease of the potential gradient with
altitude with a scale height of 6 km, we arrive at a net charge density of
∼4×10-16 C m-3 at an altitude of 35 km. Horizontal
winds, associated with atmospheric gravity waves, for example, can transport
this charge by advection and thereby drive a current. As the charge density
does not vary with time, based on the current continuity principle, we expect
the divergence of current in the horizontal direction or, in particular, the
current itself to vanish. The latter requirement results in the appearance of
polarization electric field that would ensure current balance (refer to
Eqs. 1 and 2 in D'Angelo, 1990). The relevant expression for the electric
field is E=ρU/σ, where ρ is the charge density, U is
the wind and σ is the conductivity. For a conductivity of 8×10-11 S m-1 at 35 km and for a wind speed of 50 m s-1, the
electric field generated in this way would be ∼ 0.25 mV m-1,
which is indeed small. However, it is possible that the sources are located
at lower altitudes and the fields generated therein map upward to the balloon
float altitude with an attenuation factor that would depend on the horizontal
scale size of the generated field. For scale sizes greater than a few hundred
kilometers, the attenuation factor simplifies to e-Δz/H, where
Δz is the distance from the balloon altitude to where the field is
generated (D'Angelo, 1990). Considering the source region to be located at
∼ 10 km and for a wind speed of 10 m s-1, the field generated
at the source location would be ∼ 1 V m-1. This field, when
mapped to 35 km with an attenuation factor of 0.015, would take up a value
of ∼ 15 mV m-1. Thus, even with their modest phase speeds of
10 m s-1, gravity waves of horizontal wavelengths of few hundreds of
kilometers and time periods in the range of 1–3 h can generate electric
fields of sufficient magnitude to be observable at the balloon float altitude
of ∼ 35 km. However, it remains to be seen whether such fields are a
persistent feature of the stratosphere at low latitudes.
Conclusion
The BEENS experiment was carried out from the National Balloon Facility at
Hyderabad just prior to the early hours of 14 December 2013. The primary goals
of the experiment were to observe stratospheric electric fields at these low
latitudes and understand their relationship to the known electric field
drivers. The measurements themselves had to be carefully analyzed, as there
were shortcomings like the kind of deployable boom used for vertical electric
field measurements and the probe geometry adopted, which restricted useful
electric field observations to those by one pair of horizontal probes out of
two. A malfunctioning of the orientor further restricted the observation
duration to 90 min at the float altitude of 35 km. With the apprehension
that the orientor, when powered, possibly introduced a noise field, we had to
use a small part of the data set corresponding to times when the power supply
to the orientor was switched off through a ground command. In spite of these
shortcomings, the presence of a moderate ∼ 5 mV m-1 horizontal
electric field at the float altitudes of ∼ 35 km during the flight
could be detected. The experiment had presented us several challenges to be
met, and it is hoped that future long-duration flights from these low latitudes
will be more scientifically rewarding and offer us enough scope to
investigate the role played by atmospheric sources in the generation of
electric fields of magnitudes such as reported in this work.
Data availability
The electric field data used in this work were acquired by the Indian
Institute of Geomagnetism (IIG) from a specially designed balloon experiment
conducted at the National Balloon Facility (NBF), Tata Institute of
Fundamental Research (TIFR), Hyderabad. IIG has a data sharing policy in
place, and any request for data can be addressed in line with this policy. The
thermal infrared image shown in Fig. 7 is from the archives of the NEODAAS
Dundee Satellite Receiving Station, which used the EUMETCast distribution
service provided by EUMETSAT.
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors gratefully acknowledge TIFR for providing the launch platform and necessary facilities
at its Balloon Facility at Hyderabad for conducting this experiment. All
balloon operations were carried out by the staff of NBF. The authors are
thankful to Robert Holzworth for his useful comments on an earlier design of
the payload. The BEENS experiment was carried out with the financial support
extended by the Department of Space, Government of India. S. Manu thanks the
director of the Indian Institute of Geomagnetism, Navi Mumbai, for a research
scholarship, during his association with the Indian Institute of
Geomagnetism. The topical editor, K.
Shiokawa, thanks two anonymous referees for help in evaluating this paper.
References
Bloomfield, P.: Fourier Analysis of Time Series: An Introduction, John Wiley
& Sons, Inc., 2000.
D'Angelo, N.: Comment on “A new source of horizontal electric fields in the
mid-latitude stratosphere” by R. H. Holzworth, J. Geophys. Res., 95,
11913–11914, 1990.
Franz, R. C., Nemzek, R. J., and Winckler, J. R.: Television image of a large
upward electrical discharge above a thunderstorm system, Science, 264,
48–51, 1990.
Garg, S. C., John, T., Zalpuri, K. S., Subrahmanyam, P., and Somayajulu, V.
V.: Measurement of stratospheric electrical conductivity using balloon-borne
Langmuir probe, Indian J. Radio Space, 18, 285–289, 1989.
Gonzalez, W. D., Pereira, A. E. C., Golnzalez, A. L. C., Martin, I. M.,
Dutra, S. L. G., Pinto Jr., O., Wygant, J., and Mozer, F. S.: Large
horizontal electric fields measured at balloon heights of the Brazilian
magnetic anomaly and association to local energetic particle precipitation,
Geophys. Res. Lett., 9, 567–570, 1982.
Gupta, S. P. and Narayan, A.: Balloon-borne measurements of ion conductivity
over low latitude stratosphere, Planet. Space Sci., 35, 439–443, 1987.
Hays, P. B. and Roble, R. G.: A quasi-static model of global atmospheric
electricity, 1. The lower atmosphere, J. Geophys. Res., 84, 3291–3305,
1979a.
Hays, P. B. and Roble, R. G.: A quasi-static model of global atmospheric
electricity, 2. Electrical coupling between the upper and lower atmosphere,
J. Geophys. Res., 84, 7247–7256, 1979b.
Holzworth, R. H.: A new source of horizontal electric fields in the
mid-latitude stratosphere, J. Geophs. Res., 94, 12795–12802, 1989.
Holzworth, R. H.: Quasistatic electromagnetic phenomena in the atmosphere and
ionosphere, in Handbook of Atmospheric Electrodynamics, Vol. 1, CRC Press,
Inc., 1995.
Holzworth, R. H. and Bering III, E. A.: Ionospheric electric fields from
stratospheric balloon-borne probes, in: Measurement Techniques in Space
Plasmas: Fields, Geophys. Monogr. Ser., Vol. 103, edited by: Pfaff, R. F.,
Borovsky, J. E., and Young, D. T., 79–84, AGU, Washington, D. C., 1998.
Hu, H. and Holzworth, R. H.: An inertial wave-driven stratospheric horizontal
electric field: New evidence, J. Geophys. Res., 102, 19717–19727, 1997.
Jawahar, K., Manu, S., and Gurubaran, S.: A differential electrometer for
vector electric field measurements on a balloon platform, Current Sci., 111,
624–626, 2016.
Kelley, M. C.: In The Earth's Ionosphere: Plasma Physics and Electrodynamics,
International Geophysics Series, Vol. 96, Academic, 576 pp., 2009.
Mozer, F. S. and Serlin, S., Magnetospheric electric field measurements with
balloons, J. Geophys. Res., 74, 4739–4754, 1969.
Ogawa, T., Tanaka, Y., Huzita, A., and Yasuhara, M., Horizontal electric
fields in the middle latitude, Planet. Space Sci., 23, 825–830, 1975.
Park, C. G.: Downward mapping of high-latitude ionospheric electric fields to
the ground, J. Geophys. Res., 81, 168–174, 1976.Rycroft, M. J. and Odzimek, A.: Effects of lightning and sprites on the
ionospheric potential, and threshold effects on sprite initiation, obtained
using an analog model of the global atmospheric electric circuit, J. Geophys.
Res., 115, A00E37, 10.1029/2009JA014758, 2010.
Rycroft, M. J., Nicoll, K. A., Aplin, K. L., and Harrison, R. G.: Recent
advances in global electric circuit coupling between the space environment
and the troposphere, J. Atmos. Sol.-Terr. Phy., 90–91, 198–211, 2012.
Sampath, S., Murali Das, S., and Sasi Kumar, V.: Electrical conductivities,
ion densities and mobilities in the middle atmosphere over India – Balloon
measurements, J. Atmos. Terr. Phys., 51, 533–540, 1989.
Sentman, D. D., Wescott, E. M., Osborne, D. L., Hampton, D. L., and Heavner,
M. J.: Preliminary results from the Sprites94 Aircraft Campaign: 1. Red
sprites, Geophys. Res. Lett., 22, 1205–1208, 1995.
Udare, R. S., Rajaram, R., Ogawa, T., and Yashuhara, M.: Height profile of
vertical electric field and conductivity over Hyderabad, Indian J. Radio
Space, 20, 307–309, 1991.Volland, H.: Mapping of the electric field of the Sq current into the
lower atmosphere, J. Geophys. Res., 77, 1961–1965, 1972.